This invention relates to tunable oscillators, and particularly to wideband, electrically tunable oscillators.
Electrically tunable oscillators are widely used in electronic equipment, including communications, test and instrumentation equipment. It is often desirable in the use of such equipment to closely examine a particular band of frequencies, for example, to look for spurious signals generated in a particular frequency range by a piece of RF equipment. Such an examination typically entails gradually varying the tuned frequency of, for example, a spectrum analyzer, to cover the entire band of interest. As the tuned frequency crosses from one frequency range of the instrument to another, discontinuities, or frequency jumps, commonly occur which make it difficult to track a signal of interest.
U.S. Pat. No. 4,336,505 to Meyer discloses a controlled frequency signal source apparatus including a feedback path around a VCO for the reduction of VCO phase noise in which the feedback network includes a time delay network connected to one input port of a phase detector, with the VCO output signal being supplied to the time delay network and the second input port of the phase detector. The time delay network includes a time delay and a variable phase shifter. The feedback network supplies a negative feedback signal proportional to the internally generated phase noise of the VCO to the VCO frequency control terminal.
The purpose of the Meyer feedback loop is to reduce phase noise without substantially altering the VCO tuning characteristics. In every disclosed embodiment, Meyer has a primary frequency control for the VCO other than the control provided by the feedback network. Meyer discloses a conventional phase-locked loop, including a divide-by-N counter, a frequency/phase detector, an integrator and a loop filter, for control of the VCO output frequency. In FIG. 9, Meyer discloses the use of digitally encoded frequency information to control both the frequency of the VCO and the time delay of the time delay network. As an alternative embodiment to that shown in FIG. 8, Meyer describes the use of the VCO frequency control signal which is supplied to the divide-by-N counter 62 as a signal which can be processed and supplied to digital phase shifter 80 to cause that phase shifter to supply the appropriate amount of phase shift to keep variable phase shifter 32 in its operating range. Thus, Meyer teaches the use of a feedback network employing a time delay network for reduction of phase noise but not for control of oscillator output frequency. A separate, conventional phase-locked loop is employed in conjunction with the feedback network described above for control of the VCO frequency.
U.S. Pat. No. 3,614,649 to Gerig discloses a VCO with a varactor phase shifter and a delay line connected in series with the output of the oscillator. In a fine tuning mode, a phase discriminator measures the phase difference between the output of the delay line and the output of the VCO and feeds back a difference signal through a conditioning amplifier to the input of the VCO to dynamically stabilize the oscillator frequency. When the VCO is stabilized at one of the positive-going X-axis crossings of the discriminator response characteristic, the VCO output frequency can be fine tuned over a range of 14 KHz by varying the control input voltage to the varactor phase shifter from -5 volts to +5 volts causing the phase shifter to produce a 360.degree. variation in phase shift. Gerig does not describe continuous tuning from one X-axis crossing to the next under control of the varactor phase shifter; instead, for wideband tuning, the loop is opened and the VCO is slewed in frequency.
Smooth, closed-loop tuning over multiple 14 KHz ranges is not possible with the Gerig system since a full-scale swing in the control voltage of the varactor phase shifter would be required each time a range limit of the phase shifter was reached. The full-scale voltage swing would result in a perceptible jump in frequency at each 360.degree. crossover due to voltage source limitations, such as the inability to make a large voltage change sufficiently rapidly and precisely. Such discontinuities could cause an instrument operator to lose track of a signal of interest, for example, a spurious signal generated by an RF device under test on a spectrum analyzer. A truly continuous tuning system, i.e., one capable of precise tuning over a wide range without discontinuities such as those just described, would provide a significant benefit to operators of test and instrumentation equipment such as spectrum analyzers and signal generators.
A digital phase shifter employing a read-only memory for phase angle data coupled to two multiplying D/A converters is disclosed in D. Sheingold (Ed.), Analog-Digital Conversion Handbook, Analog Devices, Inc., 1972, p. 1 -66. The description of the phase shifter does not include any reference to oscillator applications.
A number of other circuit arrangements have been designed in attempts to provide improvements in oscillators and related circuits, as illustrated by the following patents.
Lewis et al., in U.S. Pat. No. 3,936,765, discloses a surface acoustic wave oscillator comprising a surface acoustic wave delay line and an amplifier. An object of the Lewis et al. invention is to eliminate the effect of any temperature-dependent phase shift through the amplifying circuit so that the frequency of oscillation is determined by the delay line alone. The oscillator network includes a phase-sensitive detector and a variable phase shift network.
U.S. Pat. No. 4,070,635 to Healey discloses a delay line oscillator with a phase-locked loop for reducing phase fluctuations. The oscillator includes a delay line connected in series with the voltage controlled phase modulator.
U.S. Pat. No. 4,078,210 to Lewis discloses a multi-moded acoustic wave oscillator comprising a SAW delay line and a rotary switch in the feedback loop of an amplifier. Rotation of the wiper of the rotary switch changes the frequency of oscillation.
Ultrahigh frequency single mode oscillation controlled by a circuit acoustic wave crystal is described in U.S. Pat. No. 3,855,548 to Nandi et al. A surface acoustic wave device connected in a feedback path around an amplifier is the primary frequency control component of the oscillator. The feedback path includes a phase adjusting network for making small adjustments in the frequency of oscillation of the oscillator.
U.S. Pat. No. 4,358,741 to Nardin discloses a micro time and phase stepper which uses a programmable delay generator to provide selected time delays in the output signal of an oscillator.